a) Field of the Invention
The invention relates to a liquid crystal display device (LCD) for display utilizing electrooptical anisotropy of liquid crystal, and more particularly to a reflective liquid crystal display device.
b) Description of the Related Art
LCDs have advantages in that they are small, thin, and consume little power, and are therefore widely put to practical use in OA and AV equipment. Especially, active matrix LCDs having a thin film transistor (hereinafter referred to as TFT) as a switching element can perform static driving at a duty ratio of 100% in multiplex and are used for large-screen, high-resolution animation display.
TFTs are field effect transistors and are commonly disposed in matrix patterns on a substrate and connected to a display electrode configuring one of electrodes of a pixel capacitor with the liquid crystal as a dielectric layer. TFTs disposed on the same row are turned on/off simultaneously by a gate line, while a pixel signal voltage is applied through a drain line. Thus, the pixel capacitor connected to the TFTs which are designated in a matrix and turned on respectively, the displaying voltage is applied between the electrodes of the pixel capacitor according to the display content, and the pixel capacitor is charged. The display electrode and the TFT are formed on the same substrate, a common electrode configuring the other electrode of the pixel capacitor is formed on the entire plane of the other substrate which is disposed to oppose the former substrate with the liquid crystal layer intervened between them. In other words, the liquid crystal and the common electrode are divided by the display electrode, and each division forms each display pixel. The pixel capacitor is charged while the TFT is on, and the voltage which is kept by the capacitor is kept insulated by the off-resistance of the TFT for a duration of one field until the TFT is next turned on. The liquid crystal has electrooptical anisotropy and its optical properties are controlled according to the voltage applied to the pixel capacitor. By controlling a transmission factor of each display pixel, lightness or darkness of each display pixel creates displayed images.
The liquid crystal has its initial align state flexibly fixed by an alignment layer formed on the interfaces between the liquid crystal layer and both substrates.
LCD having a nematic phase liquid crystal with a negative anisotropy of dielectric constant includes a so-called VAN (vertically aligned nematic) type LCD having a vertical alignment layer for the alignment layer. VAN is one of the electrically controlled birefringence (ECB) modes and uses a difference between refractive indexes of ordinary and extraordinary axes of the liquid crystal molecules, namely birefringence, to control a transmittance. In a VAN type LCD, when a voltage is applied, an incident linear polarized light, which has penetrated one of the cross disposed polarizing plates, is birefracted in the liquid crystal layer into an elliptic polarized light. Retardation, the difference of phase velocity between ordinary and extraordinary ray components in the liquid crystal, is controlled according to an electric field strength of the liquid crystal layer and light is emitted from another polarizing plate at a desired transmittance. When the applied voltage is increased from a state with no application thereof, display changes from black to white. Therefore, the display type is a normally black mode.
As described above, liquid crystal display devices consume little power because they are prepared into a precondition ready to display by controlling the optical properties of the liquid crystal layer for each pixel by the voltage control. However, to make a display screen formed by the liquid crystal display device actually visible, a backlight is usually provided on the opposite side of the display device from that viewed in order to recognize the display pixel from transmitted light, thereby making the display pixel formed on the transparent substrate visible. The resulting disadvantage that the backlight consumes high power, means that the low power advantage of these LCDs can not be fully taken advantage of.
Consequently, there has been developed a reflective type liquid crystal display device in which a reflector is disposed on the back of the liquid crystal display device or the display electrodes is made of a material having a high reflectance to make the reflective electrode visible by utilizing environmental light, thereby making the display screen visible. Such a reflective liquid crystal display device does not require a backlight, and its power consumption is greatly lowered. It is, however, then necessary to enhance brightness and a contrast ratio in order to improve a display quality level.
It is an object of the present invention to suitably control the alignment of liquid crystal of a reflective liquid crystal display device in order to enable high quality display over a wider viewing angle.
The invention was completed to achieve the above-described object and relates to a reflective liquid crystal display device, which comprises a liquid crystal sandwiched between first and second substrates, a transparent electrode and a reflective electrode for driving the liquid crystal formed on the mutually opposed inside faces of the respective substrates, and a polarizing plate formed on the outside face of one of the first or second substrates, wherein the liquid crystal has an optical axis in its initial align state controlled into directions of the normal lines of the substrates; and, when a voltage for driving the liquid crystal is applied between the electrodes, the liquid crystal is tilted to have an azimuth of the optical axis on a plane component of the substrate form an angle of 45 degrees with respect to a polarization axis direction of the polarizing plate.
Thus, the reflectance is adjusted very accurately and gradational display is achieved by the voltage control.
The contrast ratio and response characteristic are also improved and the viewing angle can be increased by dividing the alignment of liquid crystal within the pixel.
In another aspect of the present invention, a vertical alignment layer, which controls the direction of the optical axis in the initial align state of the liquid crystal, is formed on the respective opposed inside faces of the two substrates; and the vertical alignment film is rubbed previously in order to tilt the initial alignment slightly from the directions of the normal lines of the substrates, and the liquid crystal, when the voltage for driving the liquid crystal is applied, is controlled so that the azimuth that the optical axis of the liquid crystal is inclined forms an angle of 45 degrees with respect to the polarization axis direction of the polarizing plate.
The application of a voltage for driving the liquid crystal physically controls the liquid crystal to direct its orientation into a predetermined direction and also tilt with respect to the plane face of the substrate so to enable modulation of the incident light.
In another reflective liquid crystal display device of the invention, the transparent electrode has an orientation control window not having any electrode; and, when the voltage for driving the liquid crystal is applied, the azimuth that the tilting optical axis of the liquid crystal is controlled to be 45 degrees with respect to the polarization axis direction of the polarizing plate by a non-electric field or a weak electric field produced by the orientation control window and an electric field which is produced in an inclined direction at the edges of the reflective electrode separately formed for each display pixel.
The orientation control window is formed to substantially oppose the center of the opposed face of the reflective electrode. Otherwise, the orientation control window crosses the opposed region at the center of the reflective electrode and fixes disclination of the liquid crystal into a predetermined shape in a region where the reflective electrode is formed.
As described above, by a non-electric field or a weak electric field produced by the orientation control window and an electric field which is produced in a slanting direction in the edges of the reflective electrode, the form of an electric field applied to the liquid crystal for each reflective electrode-forming region is adjusted and variations in shape of the disclination for the respective reflective electrode-forming region is prevented. The application of the voltage for driving the liquid crystal between the electrodes controls physically the respective liquid crystal molecules to direct the orientation of them into a predetermined azimuth direction and also to tilt with respect to the plane face of the substrate so to enable modulation of the incident light. Accordingly, the reflectance of the liquid crystal display device is very accurately adjusted.
According to a further aspect of the present invention, one of the two substrates, on which the reflective electrode is formed has an orientation control electrode formed in the periphery of the reflective electrode in order to generate an electric field in a horizontal direction when the voltage for driving the liquid crystal is applied; an orientation control window not having an electrode is formed in the transparent electrode; and, when the voltage for driving the liquid crystal is applied, the azimuth that the tilting optical axis of the liquid crystal is controlled to make an angle of 45 degrees with respect to the polarization axis direction of the polarizing plate by a non-electric field or a weak electric field produced by the orientation control window and an electric field in the neighborhood of the edge of the reflective electrode controlled by the orientation control electrode.
Thus, by controlling the applied voltage, the orientation controlling actions of both the orientation control window and the orientation control electrode act to forcedly adjust the shape of an electric field, the orientation of liquid crystal is controlled to have a tilt angle in an azimuth to modulate more effectively the incident light, and the reflectance can be very accurately adjusted.
Also according to the present invention, an anisotropy of electrode index xcex94n of the liquid crystal and a distance d between the first and second electrodes satisfy the following equation xcex94nxc2x7dxe2x89xa6xe2x88x920.4d+0.95. By selecting a liquid crystal material and/or configuring the liquid crystal display device so that xcex94nxc2x7d satisfies the above equation, the orientation of the liquid crystal can be controlled without fail, and a response time before the liquid crystal starts to incline when the voltage for driving the liquid crystal is applied can be made short.
Also, according to the present invention, the orientation control electrode is formed on the same plane where the reflective electrode is formed, but is formed away from the reflective electrode.
By configuring as described above, little unevenness is formed on the interfaces between the substrates and the liquid crystal layer by virtue of the presence of the orientation control window, and the liquid crystal can be prevented from having disorder in alignment. The orientation control electrode can be formed simultaneously with the reflective electrode.
The reflective crystal display device according to the present invention may further comprise thin film transistors which are separately connected to the multiple reflective electrodes and covered from above with the reflective electrodes through a planarization insulating film.
As described above, by covering the thin film transistors with the reflective electrode from their above, the alignment of liquid crystal can be prevented from being disturbed by the thin film transistors. An aperture ratio of the display device can be enlarged to its limit.